This invention relates to ultrasonic imaging techniques for measuring blood flow parameters, such as velocity, flow volume, and turbulence.
Ultrasonic imaging systems have in the past been used to assess blood flow, typically by use of Doppler techniques. True blood velocity can only be determined with Doppler techniques if the Doppler incidence angle can be determined. Additionally, flow volume is difficult to derive without making sweeping assumptions about the relation between the two-dimensional image of a blood vessel and its cross sectional dimension. Such assumptions are best made with the aid of three-dimensional imaging or other out-of-plane imaging techniques, which are often not available.
It is also known that second harmonic contrast agents can be used to enhance ultrasonic imaging of blood flow. See, for example, Uhlendorf U.S. Pat. No. 5,410,516 and Johnson U.S. Pat. No. 5,456,257. It is known that a pressure of approximately 2 MPa will destroy contrast agent such as conventional micro-bubbles, and that a much lower pressure of approximately 0.2 MPa is sufficient to produce a second harmonic response from these contrast agents for imaging purposes. Contrast agents are available commercially under the trademarks EchoGen (Sonus Pharmaceuticals, Bothell, Wash.), Optison FS069 (Molecular Biosystems Inc., San Diego, Calif.), and Levovist (Schering AG, Germany). It is customary when using harmonic imaging systems with non-linear contrast agents to manage the transmitted ultrasonic power level or pulse repetition rate so as not to destroy contrast agent bubbles excessively.